1. Technical Field
The present invention generally relates to flat fluorescent lamps and, more particularly, enhancing brightness of flat fluorescent lamps.
2. Background
It is often desirable to have a flat fluorescent lamp where brightness can be enhanced by tuning light into a smaller, yet more intense, viewable cone. Such a lamp is useful in many applications, such as displays in avionics applications and for automatic teller machine displays.
Typical flat fluorescent lamps, for example, U.S. Pat. No. 5,343,116, issued Aug. 30, 1994, to Winsor, are constructed having a substrate frit to a transparent cover lid, forming an enclosure. Diffuse channels are milled into the substrate such that they are located in the interior of the enclosure. Standard phosphors are added to the interior of the enclosure just before it is flushed with a material for emitting energy, such as argon or mercury. Energy is emitted in the form of visible light when an electric potential is introduced to the lamp.
In the past, attempts have been made to enhance flat fluorescent lamp brightness through use of reflective materials for redirecting light. U.S. Pat. No. 5,818,164, issued Oct. 6, 1998, to Winsor, discloses application of a reflective coating to the substrate exterior to reflect light back into the lamp to increase output. U.S. Pat. No. 5,479,069, issued Dec. 26, 1996, to Winsor, discloses application of reflective materials to the interior portion of the lamp to redirect light that would have been lost through rear emission. The result of applying reflective materials in this manner is to effect the forward emission of substantially all light produced by the lamp. While the addition of reflective material has enhanced the lamp efficacy, in that more light is emitted from the lamp without requiring an increase in power, these designs result in wasted light because the light is emitted in a viewable cone that exceeds the range of viewing angles required by the particular application. Therefore, even though use of reflective materials can enhance flat fluorescent lamp efficacy, a significant portion of the emitted light is wasted because it falls outside the desired viewing range. Further, the addition of reflective materials can add complexity and cost to the manufacturing process.
Attempts have also been made to enhance flat fluorescent lamp brightness by altering the shape of the diffuse channel to redirect light. For example, the flat fluorescent lamp described in U.S. Pat. No. 5,479,069 has a diffuse channel that has a symmetrical cross-section with the upper portion of the channel walls tapering outward from the diffuse channel cavity. While this type of configuration acts to enhance efficacy by directing light forward, it exits the cover lid in a viewable cone of light that is greater than the desired range of viewing angles for the application for which the lamp is designed. Therefore, even though more light is exiting the lamp, light that falls outside the desired viewing range is wasted because the outward-tapered, symmetrically-shaped diffuse channel of the prior art lamps does not enable the intensification of forward-emitted light into a focused viewing cone.
Other prior art designs have similarly utilized channel shape to force the reflected light to exit the lamp in a forward direction. Referring to FIGS. 2A-E, as light is emitted from the flat fluorescent lamp, it strikes channel wall 112 and is reflected at roughly the same angle as it approached the wall. This causes in increase in efficacy, as more light is exiting the lamp over the desired viewing angle without requiring a corresponding increase in power input. Prior art designs shown in FIGS. 2A-E are exemplary of diffuse channel 116 cross-sections that have been configured to improve flat fluorescent lamp efficacy. These figures show a diffuse channel cross-section that is symmetrical through the vertical plane and is milled so that the upper portion of channel wall 112 tapers outward away from the diffuse channel cavity 122. This outward tapering can be slight, such as FIG. 2E, or it can be extreme, as seen in FIGS. 2A-B. These symmetrical, outward-tapered diffuse channel designs assist light in being forward-emitted by having a configuration that permits light striking the channel wall to exit through cover lid 114 (as seen in FIG. 1B). This increases the efficacy of the lamp, in that overall lamp output is now greater without requiring a corresponding increase in power. However, light emitted through cover lid 114 is emitted as a cone of light that covers the full range of angles. Often, however, only a small range of the visible cone of light is needed for a particular application. Therefore, all light that falls outside that desired range is wasted because the resulting forward-emitted viewing cone of the prior art design is too large compared to the desired viewing cone for many applications.